APRS MOBILE RANGE 13 Jan 2009
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Updated 13 Jan 2009 WB4APR
In 2001 we strongly recommended in the www.aprs.org/aprs11.html
APRS1.1 addendum that all APRS software display PHG range
circles at HALF their previous value as described in the
original APRS SPEC. The SPEC value approximates the reasonable
range between two fixed stations. But it turns out that mobiles
in motion experince less than HALF this reliable range.
Mobiles in motion travel though a complex field of interacting
waves and reflections. These multiple fields add and subtract
at any instant giving from 10 to 30 dB of "mobile flutter".
When the signal is strong enough, these variances are not
noticed at all, because they are a superimposed AM modulation
to the receiver and since we use FM, these modulations are not
normally noticable until we get to weak signals. Then they
are quite noticible and pronounced as mobile flutter. This
usually occurs in multiples of 1/4 wavelengths. That is why
you can move your car only a foot or so on 2 meters and see a
change in signal from a fixed repeater.
For voice communications, a lot of mobile flutter can be
tolerated and still have good communications between humans.
A missed sylable or millisecond of voice is not noticed. But
for the mobile DIGITAL signals, and AX.25 packet, a single
millisecond hit in a packet will cause an error bit and that
will cause the entire packet to FAIL the CRC check. Thus the
packet is lost.
To account for this 10 to 30 dB of signal fluctuation, a digital
signal needs about 10 dB or more stronger signal to be reliable
than for voice. Since mobile range is proportional to the SQUARE
of the power, this means to improve the signal by 6 dB, you have
to be half as close to the repeater.
Thus, we strongly recommend that ALL APRS software display the
half-sized MOBILE PHG circles on all maps when the operator asks
for a PHG display. This gives the more realistic map of coverage
for mobiles, portables and other things on the move. Then as an
option, the operator can request a FIXED PHG display that will
display the original equation full size PHG circles.
As most people learned, the original full size PHG circles in
APRS were optimistic and so they were giving people a missleading
sense of coverage. I hope all authors will apply this HALF-SIZE
PHG fix.
Further, since there is no such thing as zero range for a radio,
and since the path between two stations is the sum of their
PHG ranges, there were some defaults that should be used for
displays for stations not reporting their PHG:
Fixed stations: PHG5130 25W, 20 feet, 3 dB gain Omni
Mobiles: PHG3020 10W, 10 feet, 2 dB gain omni
These result in range circles (half-size) of about 4 and 2 miles
respectively (8 and 4 for fixed paths according to the original
formula)..
de WB4APR, Bob